Radio Frequency Identification (RFID) systems comprise a Reader otherwise known as an Interrogator or Scanner and a plurality of Tags also known as Transponders or Electronic Labels. Systems designed for low cost applications to identify or track items may use so called Passive Tags which rely on the transmitted energy field from reader to provide their operating power and to provide a carrier signal on which they can use varying impedance modulation to signal their identity or to transmit their energy for reception by the reader. This form of modulation is commonly referred to as backscatter modulation.
Passive Tags may optionally incorporate a battery to provide energy to power the circuits on the Tag but they rely on backscatter modulation to communicate their identity. These tags are called Battery assisted Passive Tags. RFID Tagging systems employing these types of tags to communicate over ranges of more than 1 meter are designed to operate in the UHF or microwave bands, typically at frequencies in the region of 433 MHz or 860 to 960 MHz or 2.45 GHz. However these are not the only frequencies used for RFID systems.
A variety of communications protocols are used to enable the reading of multiple tags present in the operating field of the reader. These protocols are called arbitration protocols, also referred to as collision arbitration or anti-collision protocols. These protocols may use either Aloha hold-off and retry arbitration or tree walking or binary search protocols to arbitrate populations of tags present. The International Standard ISO/IEC FCD 18000 describes a number of such systems in use at different operating frequencies. Tags may be attached to items examples of which are bottles of detergent or chemicals, items of clothing, motor spares, electronic assemblies, cartons, plastic crates used in transporting perishable foods, pallets, roll cages or any other item one can imagine.
Tags may also be embedded in consumer products or other items in order to provide life cycle tracking and management. The items to which tags are attached affect the radio frequency performance of the tags or the direction from which tags can be read or the operating range of tags. In order to read tags in many situations the reader system must be configured as a portal reader in which individual reader antennas are mounted around the portal in order to provide adequate signal levels to the tags or to permit the reception of the backscatter signal from the tags. The antennas in such a system may be switched or multiplexed to provide a wider angle of radiation or greater reader volume than is possible with a single antenna as described in U.S. Pat. No. 6,367,697 Entitled “A Reader Arrangement for an Electronic Identification System Having a Plurality of Reader Heads for Energising Transponders” incorporated herein by reference. The antennas in such a system may also be combined such that all the antennas are energised simultaneously.
In the multiplexed arrangement it is possible that a number of tags out of the plurality of tags in the reader field may not receive adequate illumination of RF energy from the reader, or that the tags are arranged such that they may be illuminated by the energising signal from the reader but are in a receive null and therefore their backscatter signal from the tag is not visible to the receiver in the reader. In the combined arrangement, all antennas radiate a signal from the reader and therefore could potentially provide a solid reading field. However due to reflections and phase differences at different points in the reading volume resulting from phase changes due to propagation from the multiple antennas, energy nulls may occur in the volume and therefore tags which are placed in these nulls may never be energised or may never be visible to the receiver in the reader. It has been observed that it is possible for tags to be energised but for the response signal to not be received by the receiver in the reader.
Yet another problem with using a single reader with multiplexed multiple reader antennas results from the transmit power and receiver signal losses associated with the loss in the transmitter feedlines used to couple the reader to the antennas. In a typical 3 meter wide portal system, cables can be as long as 12 meters resulting in a power and signal loss of 6 dB or more depending on the characteristics of the feeder cable used. It is possible to reduce the loss in the cable by using coaxial cable having a larger cross sectional area, however these cables are difficult to install due to their large diameter and weight. They are also expensive and so do not provide a practical solution to the problem of cable loss.